The twin-tube tunnel, being built by the Hong Kong Highways Department, will be 1.2km long and have three lanes in either direction in the southern part of Tsing Yi from Sai Tso Wan to Nam Wan Kok. It will provide linkage between the eastern part of the New Territories and Chek Lap Kok International Airport.
On completion, the tunnel will be toll-free. The twin tunnels are 15m wide with 12 cross passages emergency walkways, and a portal building at each end and will be formed by blasting in granite and volcanic rocks.
Nearly all of the 550,000m? of tunnel spoil will be reused in the Penny's Bay reclamation project or processed into aggregates at a local quarry. Spoil will be taken away by a fleet of trucks, one leaving the site every 100 seconds during the working day from 8am to 7pm.
Environmental measures taken to minimise noise and nuisance to road users and the public include: a 7.5m-tall noise barrier, and continuous monitoring to ensure noise and vibration limits are not exceeded during tunnel blasting; blast doors for any blasting conducted near roads; and 40 water sprinklers and washing facilities at all exits for vehicles to use before leaving the construction site.
The tunnel cross section will be in the form of an arch. The approximate height and width of the arch is 11.2m and 15.3m respectively. The geology comprises coarse volcanic ash tuff on the west side and medium grained granite on the eastern side. Both of these lithologies are intruded by Rhyolite dykes together with some porphyrytic granite and occasional basalt dykes.
The two tubes are being constructed using the drill / blast method. Temporary support for the tunnels will be provided by rock-bolts. Permanent support will be provided by a concrete lining of varying thickness depending upon the stability of the ground.
Three main types of support will be used: 400mm un-reinforced, 500mm un-reinforced, and 600mm reinforced.
Ove Arup and Partners Hong Kong Ltd were the designers of the tunnel and Gammon Skanska and Skanska International Civil Engineering joint venture are the main construction contractors . The contract also covers construction of tunnel control buildings and the 1.4km dual three-lane West Tsing Yi Viaduct.
The cost for the tunnel has been estimated at HK$470m and HK$83 million . Fl?kt Woods is providing the ventilation system for the tunnel.
COWI has provided specialist assistance to Ove Arup and Partners with the design of electrical and mechanical systems for the Nam Wan Tunnel and two adjoining viaducts.
Fl?kt Woods is providing the ventilation solutions by supplying three immense fans for the tunnel. The axial flow fans measure 2.65m in diameter, are powered by 900kW motors and run at pressures of 3,800Pa.
Fl?kt Woods will run one of the fans continuously to self destruct, to prove their capability of withstanding temperatures of up to 400°C for one hour. The fans incorporate guide vanes for flow straightening to achieve maximum performance and will run at 993rpm.
Electrical and Mechanical Systems
In the preliminary design phase of the tunnel COWI established an overall concept for the electrical and mechanical systems with emphasis on safety-related functions such as electric power supply, tunnel lighting, fire fighting, tunnel ventilation and smoke extraction. This overall concept was used as a basis for the detailed design.
Later in the project COWI reviewed the client's detailed design of electrical and mechanical systems.
The electrical and mechanical systems will include the following: high- and low-voltage distribution ; reliable power supply based on UPS and diesel generators; tunnel lighting and road lighting; lighting in portal buildings, technical rooms and cross passages between tunnels; Central Monitoring and Control System ; fire detection system; public mobile communication system; fire fighting equipment, including fire water booster pumps, hydrants and portable extinguishers; tunnel ventilation for normal operation, congested operation and emergency situations ; smoke extraction system; pressurised air supply; HVAC and plumbing in portal buildings.